The present invention concerns a process for the industrial synthesis of urea, making ammonia and carbon dioxide react in at least one reaction space, at high pressure and temperature, and recycling at least partially the unreacted products.
The invention concerns also the applications of this process to conventional systems in order to improve the urea yield and reduce energy consumptions.
New or modified plants obtained by the implementations of the above mentioned process and its applications to pre-existing systems, are within the scope of the invention.
Known are several types of processes, systems and plants for the industrial synthesis of urea from NH3 (in excess) and CO2, with recycling after stripping. Among the most prominent of these is the Snamprogetti process of isobaric stripping with NH.sub.3.
The main technical characteristics of the isobaric stripping process (Snamprogetti) can be resumed as follows:
synthesis pressure: approx. 150 bar PA1 NH3/CO2 mol in the reactor: approx. 3.2+3.4 PA1 H2O/CO2 mol in the reactor: approx. 0.6+0.7 PA1 temperature of the reactor: 190.degree. C. PA1 yield: approx. 62+63% PA1 vapour consumption: approx. 900 kg/MT urea
The above indicated values are rather consolidated and drastic improvements of the process do not seem to be possible.
Among the other processes with separate recycle of NH3 widely used in the past can be mentioned the ones of Toyo Engineering Ltd. indicated with the abbreviation TRC-TRC-I (Total Recycle C, TRC Improved), characterized by the fact that the ratio NH3/CO2 in the reactor is equal to approx. 4+4.5 mol and the separation of NH3 at 18+20 bar is more impressive than the previous process (Snamprogetti), also for the lack of a isobaric stripping.
Another generation of processes has been recently developed for increasing the yield in the reactor of the so called stripping processes (Snamprogetti NH3 selfstripping and Stamicarbon CO2 stripping), of the conventional values 55+65% up to 70+75%, providing at the same time, the above mentioned high yield reactors with isobaric loops.
Reference is made to the new and more recent processes as "IDR" (Isobaric Double Recycle) of Montedison and "ACES" of Toyo Engineering Ltd.
In the above mentioned recent processes "IDR" and "ACES", still operating with high ratios NH3/CO2 in the high yield reactor, all the unaltered reactants, including the high excess of NH3, are recycled to the reactor under the form of carbamate solution (the separate recycling of NH3 does not exist).
Without doubt much better yields are obtained (compared with the very first processes), but these advantages are coupled with a high complexity and complications in the construction of the equipment.
Furthermore these processes have the inconvenience of having to strip the unreacted products under high temperature conditions with real risks of decomposition of the (laboriously) developed urea.
The first aim of the present invention is to provide a process without the above mentioned inconveniences, which can be operated easily, with total high yields and low consumptions.
Another aim of the invention is the applicability of this process to the conventional systems already having recycling sections for ammonia (or adapted with a new recycling section of the ammonia), reaching global yields of urea transformation in the synthesis sections equal or higher to the ones obtained with the modern processes "IDR", "ACES", etc.
A further aim of the invention is represented by the simple and effective plants (new or modified) resulting from the implementation of the said process.